1. Field of the Invention
This invention relates to a continuous process for extrusion of thermoplastic resin of sheets, films, pipes and profiles during the polymerization processes, more particularly to a process in which a pressure-adjusting device and at least one extrusion device are used for simplifying the extrusion process.
2. Description of the Related Art
At present, the process for manufacturing thermoplastic resin products, such as sheets, pipes, films, profiles, etc., comprises extruding a resin melt, which is produced from polymerization and devolatilization processes, into strands by means of a strand extruder. The strands are cooled by means of water cooling system or air cooling system. The cooled plastic strands are then cut into pellets by pelletizer and sieved by a sieving machine and then transferred to a storage container by means of air conveying. End users could make use of these pellets to produce thermoplastic sheets, pipes, films and profiles, etc. As in the case of making polymethyl methacrylate (PMMA) sheet and polystyrene (PS) sheet, the plastic pellets are first delivered to a mixing tank to mix with additives and pigments which form a uniform coating on the pellet surface followed by a drying step. The dried mixture is then fed to the hopper of a sheet extruder and is melted and passed through a "T die" to form the thermoplastic sheets.
As the above described, conventional the process for extrusion of thermoplastic resins of sheets has the following disadvantages:
(1) The polymerized and devolatilized plastic material must be treated by means of cooling, pelletizing, delivering and storing steps before being used for the end users, and then mixing, drying and extruding so as to form sheets, pipes, profiles etc. This process is complicated and a plurality of complicated devices or systems are utilized in order to carry out this process. As long as one of the devices is in trouble, the remaining steps of the process will be certainly interrupted. In addition, when the plastic pellets is conveying by air or mixing with additives, dust is generally produced, which is harmful to the health of operators.
(2) The invest for the tedious process, such as cooling, pelletizing, delivering, storing, drying and extruding are expensive. Problems of noise, high energy consumption of electricity, and wasted gas and water are also encountered.
(3) The process has a plurality of heat treatment steps including an additional screw extruding step, which may result in degradation of part of the resin, thereby result in a poor base color of the extrusion product.
To overcome aforementioned disadvantages, the conventional process must be simplified. That is to say, the steps of cooling, pelletizing, delivering, drying, extrusion melting must be eliminated from the process and the polymerized and devolatilized thermoplastic resin melt is directly fed into the extrusion device, such as sheeting die, to produce thermoplastic resin sheets directly. However, such a simplified process has a problem which cannot be solved at the present technical level. The problem is that, for example, in the process for extruding plastic sheets, when the polymerized and devolatilized molten resin is directly fed to the sheeting die, the sheeting process cannot be operated stably due to unsteady feeding of the molten resin. In addition, if the flow rate of the molten resin is disturbed an improper operation of extrusion device, the obtained plastic sheets will have a surface with wavy stripes. Therefore, sheets with an excellent surface quality and a uniform thickness cannot be obtained by means of the simplified process.